Other top donors giving millions of dollars to aid Romney's campaign include a trio of Texas money moguls and the head of a South Florida-based energy conglomerate. Their contribution totals expanded by $17 million in the first half of October, according to Federal Election Commission records. Adelson remained ahead of the pack, donating $10 million in October, but the others also added recently to their totals.

Those donors and others are funding a presidential election that surpassed $2 billion in October, with money going toward individual Democratic and Republican campaigns as well as independent, "super" political committees working on the campaigns' behalf.

Political donations can open doors that are closed to most people. Big-dollar donors are often invited to state dinners at the White House and other events with the president. They also may be asked to weigh in on public policy, especially if it affects their own financial interests. And the ranks of ambassadors, advisory panels and other government jobs traditionally are filled with those who have been unusually generous during the campaign.

Based on an examination of more than 3.9 million campaign contributions through final pre-election finance reports in mid-October — the methodology is below — The Associated Press has ranked the top five financial supporters bankrolling the Republican presidential run:

Adelson is the largest declared donor to the Romney campaign and supporting political committees, providing more than $44.2 million this election season. He and his wife, Miriam, a physician who heads the Nevada-based Adelson Drug Clinic, have given $20 million to the Restore Our Future, a super PAC backing Romney. The couple gave half of that total to the super PAC in early October. Adelson also joined relatives to give $24 million to committees backing former GOP presidential candidate Newt Gingrich. And he has made public pledges to give $10 million to Karl Rove's American Crossroads super PAC and as much as $100 million this election more broadly to the GOP. Worth an estimated $25 billion, Adelson oversees the Las Vegas Sands Corp., which runs casino and resort interests in Las Vegas, Singapore and Bethlehem, Pa., and Sands China Ltd., a cluster of casinos in the Chinese territory of Macau. He would benefit from loosened trade restrictions and a rise in the Chinese currency rate against the dollar. His company devoted $60,000 this year to lobby on tax issues, foreign tourist visas, travel and Internet gambling issues — and has spent $1.86 million lobbying on legislation dealing with China trade, gambling and travel since 2002. A staunch supporter of Israel, he also is a contributor to the Republican Jewish Coalition, which spent $920,000 since 2002 backing bills aimed at pressuring Iran and enhancing U.S. security cooperation with Israel. Adelson's casino company has advised shareholders that it was under investigation by the Justice Department and the Securities and Exchange Commission. Investigators were said to be focusing on the Macau casinos and reports of missing money and possible violations of the Foreign Corrupt Practices Act.

Simmons is a longtime backer of GOP and conservative causes. He has donated $22 million to the party's efforts this year, including $17.5 million to American Crossroads. Simmons gave Rove's committee $6.5 million in early October. Simmons has donated $2.3 million to Restore Our Future — $500,000 in September. Simmons and his wife, Annette, also gave $2.2 million to Super PACs backing former GOP presidential candidates Gingrich, Rick Santorum and Rick Perry. Simmons has been active in political fundraising since the 1990s and in 2004 was a $4 million backer of the Swift Vets campaign, the GOP-backed effort to discredit Democratic presidential nominee John Kerry's military record in the Vietnam War. Simmons' Titanium Metals Corp. reportedly is a top producer of titanium for weapons and other industrial uses. He also owns a majority stake in Valhi Inc., a Texas-based waste management company, and could benefit from a proposed Nuclear Regulatory Commission rule change that would allow the company's Texas facility to store spent uranium from nuclear power and weapons plants. Contran's subsidiaries have spent $200,000 this year lobbying the NRC, the Energy Department, the Senate and the House on metals and waste issues, and $4.3 million over the past decade, including efforts to protect a Pentagon rule limiting titanium purchases to U.S. producers. Simmons was fined $19,800 by the Federal Election Commission in 1993 for exceeding the then-annual $25,000 limit on individual campaign contributions, which has since been lowered.